README.md

Flutter Encryption & Decryption App

📌 Description

This is a simple Flutter app that encrypts and decrypts text using a circular shift algorithm based on a given numeric key.

🔹 How It Works

• If the key is positive, letters shift forward (A → B, Z → A).
• If the key is negative, letters shift backward (A → Z, C → A).
• Special characters & numbers remain unchanged.

🚀 How to Run

1. Click on the Appetize.io link to open the app:
   👉 Run the App

2. Follow these steps:

   • Enter text in the input field.
   • Provide a numeric key (positive or negative).
   • Click "Encrypt" to shift letters based on the key.
   • Click "Decrypt" to revert to the original text.

Enjoy using the app!

encrpytion_app.dart

import 'package:flutter/material.dart';

// Function to encrypt with circular shift
String encrypt(String plaintext, int shift) {
  String encrypted = '';

  for (int i = 0; i < plaintext.length; i++) {
    String char = plaintext[i];

    if (char.contains(RegExp(r'[A-Z]'))) {
      int newCode = ((char.codeUnitAt(0) - 65 + shift) % 26 + 26) % 26 + 65;
      encrypted += String.fromCharCode(newCode);
    } else if (char.contains(RegExp(r'[a-z]'))) {
      int newCode = ((char.codeUnitAt(0) - 97 + shift) % 26 + 26) % 26 + 97;
      encrypted += String.fromCharCode(newCode);
    } else {
      encrypted += char;
    }
  }

  return encrypted;
}

// Function to decrypt (just reverse the shift)
String decrypt(String encryptedText, int shift) {
  return encrypt(encryptedText, -shift);
}

void main() {
  runApp(EncryptionApp());
}

class EncryptionApp extends StatefulWidget {
  @override
  _EncryptionAppState createState() => _EncryptionAppState();
}

class _EncryptionAppState extends State<EncryptionApp> {
  String encryptedText = '';
  String decryptedText = '';

  final plaintextController = TextEditingController();
  final keyController = TextEditingController();

  void encryptText() {
    int shift = int.tryParse(keyController.text) ?? 0;
    setState(() {
      encryptedText = encrypt(plaintextController.text, shift);
    });
  }

  void decryptText() {
    int shift = int.tryParse(keyController.text) ?? 0;
    setState(() {
      decryptedText = decrypt(encryptedText, shift);
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
        scaffoldBackgroundColor: Colors.blueGrey[400],
      ),
      home: Scaffold(
        appBar: AppBar(
          title: const Center(child: Text('Encryption & Decryption')),
          elevation: 5,
        ),
        body: Padding(
          padding: const EdgeInsets.all(20.0),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            crossAxisAlignment: CrossAxisAlignment.stretch,
            children: <Widget>[
              TextField(
                controller: plaintextController,
                decoration: const InputDecoration(
                  labelText: 'Enter Text',
                  border: OutlineInputBorder(),
                  filled: true,
                  fillColor: Colors.white,
                ),
              ),
              const SizedBox(height: 10),
              TextField(
                controller: keyController,
                decoration: const InputDecoration(
                  labelText: 'Enter Key (positive or negative)',
                  border: OutlineInputBorder(),
                  filled: true,
                  fillColor: Colors.white,
                ),
                keyboardType: TextInputType.number,
              ),
              const SizedBox(height: 20),
              ElevatedButton(
                onPressed: encryptText,
                style: ElevatedButton.styleFrom(
                  backgroundColor: Colors.blueAccent,
                  padding: const EdgeInsets.symmetric(vertical: 15),
                  shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(10),
                  ),
                ),
                child: const Text('Encrypt', style: TextStyle(fontSize: 18)),
              ),
              const SizedBox(height: 10),
              Text(
                'Encrypted: $encryptedText',
                textAlign: TextAlign.center,
                style:
                    const TextStyle(fontSize: 16, fontWeight: FontWeight.bold),
              ),
              const SizedBox(height: 20),
              ElevatedButton(
                onPressed: decryptText,
                style: ElevatedButton.styleFrom(
                  backgroundColor: Colors.greenAccent[700],
                  padding: const EdgeInsets.symmetric(vertical: 15),
                  shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(10),
                  ),
                ),
                child: const Text('Decrypt', style: TextStyle(fontSize: 18)),
              ),
              const SizedBox(height: 10),
              Text(
                'Decrypted: $decryptedText',
                textAlign: TextAlign.center,
                style:
                    const TextStyle(fontSize: 16, fontWeight: FontWeight.bold),
              ),
            ],
          ),
        ),
      ),
    );
  }
}